Lift trucks are typically used for stacking palletized loads. However, other types of loads may be lifted by the truck using suitable handling means. Typically, lift trucks having a vertically movable fork assembly use a double mast assembly, wherein the vertical parallel masts each contain a raceway for guiding the fork assembly vertically up and down. This double mast type design provides a high degree of forward bending resistance and pallet tilting resistance and, thus, is generally incorporated in a lift truck which must have a capability to raise and lower relatively heavy loads.
For lighter loads of, for example, 200 lbs., the mast structure need not be as rugged, and a single mast may be adequate to support the fork assembly and load.
In one embodiment of a prior art single mast lifting apparatus, the single mast incorporates a raceway for guiding the fork assembly vertically up and down the mast, similar to the raceways used in double mast type lifting devices.
One example of a single mast lifting device not using a raceway is the Model EZY-150 Palletizer from FMC Corporation of Hoopeston, Ill. This particular palletizer has a specified maximum payload of 200 lbs. and is designed to pick up a case from a conveyor belt and place the case on a pallet. The general construction of this Model EZY-150 Palletizer is shown in FIG. 1.
In FIG. 1, single mast 12 sustains a torque exerted by the weight of carton 14 grasped by lifting assembly 16. Sleeve 18 is coupled to lifting assembly 16 and essentially surrounds the rectangular single mast 12. Sleeve 18 is moved up or down single mast 12 by a drive assembly (not shown) located within support base 20 and the hollow mast 12.
In supporting the load grasped by lifting assembly 16 of FIG. 1, bearing wheel pairs 22, 24, and 26 are used to provide sleeve 18 with a low friction guiding means when moving up and down mast 12. Thus, bearing wheel pairs 22, 24, and 26 apply to mast 12 the forces resulting from the weight of lifting assembly 16 and carton 14.
As seen in FIG. 1, the force provided by bearing wheel pair 22 against side 12A of rectangular mast 12 is primarily due to torque forces in the x direction, shown in FIG. 1. Bearing wheel pairs 24 and 26 provide virtually all the force against mast 12 resulting from torque forces in the z, or downward, direction. Essentially, the torque provided lifting assembly 16 and carton 14 is such that mast 12 is resisting a force to bend mast 12 toward the location of carton 14.
Since the maximum torque to be applied to a single mast, such as mast 12, determines the dimensions, thickness, material, and cost of the mast, any modification to the mast and bearing wheel assemblies to support an increased torque without requiring a more rugged mast would greatly add to the desirability and commercial aspects of a lifting apparatus incorporating such an improved mast and bearing wheel assembly.